Minerals almost invariably occur in nature in ores which contain a variety of materials in addition to the particular mineral constituent that is to be marketed. The nondesired mineral matter may be, for example, an impurity or a particle size fraction of the desired mineral that is too coarse (or too fine) for an intended use. Especially where the desired mineral material is very finely divided, for example, a material having an appreciable content of particles finer than 2 micrometers, it is conventional to disperse the ore in water to form an aqueous pulp before attempting to upgrade (beneficiate) the ore. Dispersion (deflocculation) is practiced to fluidize mineral pulps and it enhances the separation of individual mineral particles from others by increasing the electrical charge on the individual particles.
Anionic dispersants such as condensed phosphates and sodium silicate are frequently used to fluidize ore pulps containing negatively charged mineral particles at near neutral to mildly alkaline pH values (e.g., pH 6-9) without the need to consider the effect of using such dispersants on subsequent processing. An exception is the case of finely mineralized ores such as kaolin clay crudes in which chemical treatment after dispersion is frequently essential and the presence of the particular dispersant used to upgrade the ore may result in undesirable interaction between dispersant and subsequently or previously added reagents. See, for example, U.S. Pat. No. 3,594,203 (Sawyer et al).
Naturally-occurring sources of kaolin, generally known as kaolin crudes, such as those mined in Georgia, U.S.A. and Cornwall in England, are processed to recover upgraded kaolin products. Many Georgia crudes contain well-crystallized finely divided particles of kaolin having a median particle size of about 1.5 micrometers in association with silica and silicate impurities as well as colored ferruginous and titaniferous impurities. Other commercially important Georgia crudes contain less well-crystallized kaolin which are finer in particle size, e.g., median size below 1 micrometer. The Cornwall crudes typically contain only about 10% to 15% kaolin which is coarser than the kaolin in the Georgia deposits. Vast tonnages of such kaolin crudes are mined for the ultimate recovery of purified grades of kaolin having a higher brightness and different particle size distribution than that of the kaolin in the crude. The processing, frequently referred to as "wet processing", almost invariably involves crushing the kaolin crude, pulping the crushed crude in water, removing coarse impurities (so-called grit), fractionating the degritted crude to recover one or more fractions of desired particle size distribution, bleaching to brighten one or more of the fractions of kaolin, filtration and washing. In some cases, additional upgrading to remove colored impurities by flotation, selective flocculation and/or magnetic purification is practiced and usually takes place before filtration. Such additional upgrading also takes place when the clay is in the form of a dispersed pulp.
When kaolin crudes are mixed in water without addition of dispersant the resulting pulps are usually mildly acidic, typically having a pH of 4-6 and the kaolin particles are negatively charged. To the best of our knowledge, one or more anionic dispersants are invariably added to a pulp of the crude to create near neutral or mildly alkaline systems before degritting and fractionation. When these steps are followed by bleaching with a reducing bleach, usually a hydrosulfite (dithionite) salt, the previously dispersed alkaline pulp of clay is first flocculated by adding an acid or alum to pH 3-5 because such bleaches are generally more effective at acidic pH values and flocculation aids subsequent filtration. However, the added acid or alum interacts with the dispersant to form salts which are frequently deleterious to performance properties of the kaolin product. After flocculation, the bleached clay, in the form of a filter cake, must usually be washed to reduce salt content. Filtration and washing add considerably to the cost of processing. Subsequent processing depends on the form in which the purified kaolin is to be supplied. So-called "predispersed grades" are usually manufactured by adding a dispersant to the previously flocculated clay to form a high solids slurry (e.g., 65-70%) which is spray dried to provide a dry product containing a dispersant. The production of slurried grades also entails the addition of dispersant(s). Dispersant(s) added to previously dispersed and flocculated clay are generally termed "secondary" dispersants to distinguish them from dispersants used to prepare pulps for degritting, fractionation, etc., which are generally referred to as "primary" dispersants. Acid grades of wet processed kaolins are produced without secondary dispersion of acid-flocculated filtered clay.
The history of using alkali or negatively charged material as primary dispersants for negatively charged clay dates at least back to 1912 (U.S. Reissue Pat. No. 14,583, Scherwin). This patent teaches using "electrolytes containing ions of an electric character which is the same as the character of the part of the composite mass which it is desired shall remain in suspension in sol-condition". This concept is pursued in Scherwin's subsequent patent (U.S. Pat. No. 1,233,713), which discloses fractional sedimentation of kaolin in a dispersed system. An advance is reflected in U.S. Pat. No. 1,324,958 (Feldenheimer) in which an acidic material is used to neutralize the alkaline deflocculating agent and to floc the clay in order to settle materials not ordinarily settled in a deflocculated condition. Subsequent patents disclose particle size fractionation of kaolin in dispersed state in centrifuges, usually followed by flocculation by acid addition, filtration and washing of the filter cake. A major advance in the development of the kaolin industry was reflected in the establishment of a 2 micrometers particle size cut-off point when fractionating kaolins to prepare paper coating clay and the use of centrifuges to carry out the desired fractionation of the crude clay. Crudes having a median particle size of 1.5 micrometers are conventionally classified in centrifuges to recover premier coating grades (at least 90% by weight finer than 2 micrometers) and coarse filler kaolin having a median size of 4-6 micrometers. A major present use of kaolin crudes is as a source of bleached minus 2 micrometers fractions of kaolin clay supplied principally for the paper coating industry.
The growth of the kaolin industries in the U.S. and abroad has also generated many other innovations including the use of a variety of primary dispersants, all anionic, and including sodium silicates, silicate hydrosols, condensed phosphate salts, polyacrylate salts, and "auxiliary" dispersants such as sulfonates.
Similarly, the art of producing carbonate pigments and fillers from naturally-occurring dolomitic and calcitic ores by processes utilizing wet milling and wet particle size classification has used anionic dispersants, exemplified by sodium polyacrylates, added, for example, during grinding, followed in some instances by flocculation. See, for example, U.S. Pat. No. 3,980,240 (Nott), U.S. Pat. No. 3,990,642 (Nott), and U.S. Pat. No. 4,165,840 (Lewis et al).
The concept of using a negatively charged dispersant (sodium silicate, mixtures thereof with sodium carbonate, or hydrosols derived from sodium silicate) in the froth flotation of kaolin has been extended to other finely mineralized negatively charged mineral oxide and silicate ores, for example, talc ores, tin (cassiterite) ores, (U.S. Pat. No. 3,915,391, Mercade), fluorspar ores (U.S. Pat. No. 3,893,915, Mercade) and scheelite (tungsten) ores, (U.S. Pat. No. 3,915,391, Mercade).
Cationic surfactants have been disclosed as a means to disperse various clays in organic liquids. See, for example, U.S. Pat. No. 2,797,196, Dunn et al.
In accordance with U.S. Pat. No. 3,804,656, Kaliski, et al, a negatively charged filler or pigment such as kaolin is dispersed with a combination of nonionic and cationic surface active agents at a strongly alkaline pH. The cationic material is not added as a primary dispersant nor is it present during wet processing.
U.S. Pat. No. 4,738,726, Pratt et al, discloses high bulking pigments obtained by adding a cationic polyelectrolyte to previously degritted and fractionated kaolin clay to partially flocculate the clay. Anionic primary and secondary dispersants are used and the cationic material is used to partially flocculate, not to disperse the clay.
U.S. Pat. No. 4,210,488 (Reus et al) discloses addition of a cationic polymer (polypiperidine halide) "absorbed" on a fine particle carrier such as kaolin to improve the dry strength of paper or to improve the effect of optical brighteners. The absorbed polymer is said to create a positive charge on the kaolin particles. The cationic material is not added during wet processing of the carrier.
Numerous publications make reference to the addition of cationic polyelectrolytes to paper coating composition containing mineral pigments such as clay or calcium carbonate to improve the coating structure by imparting a bulked structure. See, for example, Coco, C.E., Soy Protein Latex Interpolymers--Properties and Function, Preprints TAPPI Coating Conference 1987.
EPA 281,134 (Weige) relates to cationic pigment dispersion, especially for producing paper coating compositions, containing a pigment component, a cationised polymer, which encloses the pigment particles as a protective colloid, and optionally a cationic polymer and/or ammonium compound as dispersant for the coated pigment particles. The cationic polymer PVA increases the zeta potential of the dispersion to the isoelectric point or into the cationic region. The pigment can be ground or precipitated calcium carbonate, (calcined) kaolin, TiO.sub.2, ZnO, satin white, aluminum hydrosilicates or their mixtures.
In commonly assigned copending application U.S. Ser. No. 07/347,612, filed concurrently herewith, cationically processed kaolin crude of the type described in U.S. Pat. No. 3,586,523 (Fanselow et al) is calcined to provide an improved high brightness, low abrasion kaolin opacifying pigment useful for coating or filling paper or an ingredient of paints or plastics. Our copending patent application, U.S. Ser. No. 07/309456 filed Feb. 10, 1989, describes the use of cationic polymers and the like as secondary dispersants for slurrying calcined kaolin pigments.
To the best of our knowledge, the prior art does not disclose or suggest the use of cationics as primary dispersants for kaolin or other ores.